Fluid propulsion devices achieve thrust by imparting momentum to a fluid called the propellant. An air-breathing engine, as the name implies, uses the atmosphere for most of its propellant. The gas turbine produces high-temperature gas which may be used either to generate power for a propeller, fan, generator or other mechanical apparatus or to develop thrust directly by expansion and acceleration of the hot gas in a nozzle. In any case, an air breathing engine continuously draws air from the atmosphere, compresses it, adds energy in the form of heat, and then expands it in order to convert the added energy to shaft work or jet kinetic energy. Thus, in addition to acting as propellant, the air acts as the working fluid in a thermodynamic process in which a fraction of the energy is made available for propulsive purposes or work.
Typically turbofan engines include at least two air streams. All air utilized by the engine initially passes through a fan, and then it is split into the two air streams. The inner air stream is referred to as core air and passes into the compressor portion of the engine, where it is compressed. This core air then is fed to the combustor portion of the engine where it is mixed with fuel and the fuel is combusted. The combustion gases then are expanded through the turbine portion of the engine, which extracts energy from the hot combustion gases, the extracted energy being used to run the compressor, the fan and other accessory systems. The remaining hot gases then flow into the exhaust portion of the engine, which may be used to produce thrust for forward motion to the aircraft.
The outer air flow stream bypasses the engine core and is pressurized by the fan. Typically, no other work is done on the outer air flow stream which continues axially down the engine but outside the core. The bypass air flow stream also can be used to accomplish aircraft cooling by the introduction of heat exchangers in the fan stream. Downstream of the turbine, the outer air flow stream is used to cool engine hardware in the exhaust system. When additional thrust is required (demanded), some of the fan bypass air flow stream may be redirected to the augmenter (afterburner) where it is mixed with core flow and fuel to provide the additional thrust to move the aircraft, in some applications
Many current and most future aircraft need efficient installed propulsion system performance capabilities at diverse flight conditions and over widely varying power settings for a variety of missions. Current turbofan engines are limited in their capabilities to supply this type of mission adaptive performance, in great part due to the fundamental operating characteristics of their core systems which has limited flexibility in load shifting between shaft and fan loading.
When defining a conventional engine cycle and configuration for a mixed mission application such as a mixed turbofan and turboshaft application, compromises have to be made in the selection of fan pressure ratio, bypass ratio, and overall pressure ratio to allow a reasonably sized engine to operate effectively. In particular, the fan pressure ratio and related bypass ratio selection needed to obtain a reasonably sized engine capable of developing the thrusts needed for combat maneuvers are non-optimum for efficient low power flight where a significant portion of the engine output is transmitted to the shaft. In some applications, it is desired to reduce engine thrust in order to transfer more power to a shaft which drives a lift rotor, propeller, generator, or other device or system external to the turbofan engine.
Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views, FIG. 1A shows a general orientation of a turbofan engine in a cut away view. In the turbofan engine shown the flow of the air is generally axial. The engine direction along the axis is generally defined using, the terms “upstream” and “downstream” generally which refer to a position in a jet engine in relation to the ambient air inlet and the engine exhaust at the back of the engine. For example, the inlet fan is upstream of the combustion chamber. Likewise, the terms “fore” and “aft” generally refer to a position in relation to the ambient air inlet and the engine exhaust nozzle. Additionally, outward/outboard and inward/inboard refer to the radial direction. For example the bypass duct is outboard the core duct. The ducts are generally circular and co-axial with each other.
As ambient inlet airflow 12 enters inlet fan duct 14 of turbofan engine 10, through the guide vanes 15 and passes by fan spinner 16, through fan rotor (fan blade) 42. The airflow 12 is split into primary (core) flow stream 28 and bypass flow stream 30 by upstream splitter 24 and downstream splitter 25. In FIG. 2, the bypass flow stream 30 along with the core/primary flow stream 28 is shown, the bypass stream 30 being outboard of the core stream 28. The inward portion of the bypass steam 30 and the outward portion of the core streams are partially defined by the splitters upstream of the compressor 26. The fan 42 has a plurality of fan blades.
As shown in FIGS. 1A and 1B the fan blade 42 shown is rotating about the engine axis into the page, therefor the low pressure side of the blade 42 is shown, the high pressure side being on the opposite side. The primary flow stream 28 flows through compressor 26 that compresses the air to a higher pressure. The compressed air typically passes through an outlet guide vane to straighten the airflow and eliminate swirling motion or turbulence, a diffuser where air spreads out, and a compressor manifold to distribute the air in a smooth flow. The core flow stream 28 is then mixed with fuel in combustion chamber 36 and the mixture is ignited and burned. The resultant combustion products flow through turbines 38 that extract energy from the combustion gases to turn fan rotor 42, compressor 26 and any shaft work by way of turbine shaft 40. The gases, passing exhaust cone, expand through an exhaust nozzle 43 to produce thrust. Primary flow stream 28 leaves the engine at a higher velocity than when it entered. Bypass flow stream 30 flows through fan rotor 42, flows by bypass duct outer wall 27, an annular duct concentric with the core engine, flows through fan discharge outlet and is expanded through an exhaust nozzle to produce additional thrust. Turbofan engine 10 has a generally longitudinally extending centerline represented by engine axis 46.
A typical turbofan engine employs a two-shaft design, with a high-pressure turbine and the compressor 26 connected via a first shaft and a low-pressure turbine and the fan blade 42 connected via a second shaft. In most designs the first and second shafts are concentrically located.
In most turbofan engines a significant portion of the engine's thrust is produced by the rotation of fan blades 42 to create airflow in the bypass stream 30. However, as noted above in some applications it is desirable to reduce an engine's thrust in order to transfer power to other systems, devices, or applications. Thus, an effective means is needed to reduce a turbofan engine's thrust while maintaining overall power produced by the core.
These and many other advantages of the present subject matter will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of preferred embodiments.
The present application discloses one or more of the features recited in the appended claims and/or the following features which, alone or in any combination, may comprise patentable subject matter.
While the present disclosure is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. It should be understood, however, that the present disclosure is not intended to be limited to the particular forms disclosed. Rather, the present disclosure is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the disclosure as defined by the appended claims.